An optical fiber is conventionally constituted of an optical core, which transmits an optical signal, and of an optical cladding, which confines the optical signal within the optical core. To that end the refractive index of the core, nC, is greater than the one of the cladding, nCl. An optical fiber is generally characterized by a refractive index profile that associates the refractive index (n) with the radius (r) of the optical fiber: the distance r with respect to the center of the optical fiber is shown on x-axis and the difference Dn between the refractive index at radius r, n(r), and the refractive index of the optical cladding nCl is shown on y-axis.
Nowadays, two main categories of optical fibers exist: multimode fibers and single-mode fibers. In a multimode fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber, whereas in a single-mode fiber, the higher order modes (hereafter called HOMs) are cut-off or highly attenuated.
Single-mode fibers are commonly used for long-distance applications, such as access networks or metropolitan networks. To obtain an optical fiber capable to transmit a single-mode optical signal, a core with a relatively small diameter is required (typically between 5 μm and 11 μm). To meet requirements of high speed or bit-rate applications (for example 10 Gbps), standard single-mode fibers require use of a modulated single-mode laser emitter tuned to work typically at a wavelength of 1550 nm. However, single-mode fibers suffer from nonlinearly problems, which are major limitations on fiber transmission capacity.
Multimode fibers are commonly used for short-distance applications requiring a high bandwidth, such as local area networks (LANs) and multi-dwelling units (MDUs), more generally known as in-building networks. The core of a multimode fiber typically has a diameter of 50 nm, or 62.5 μm. The most prevalent multimode fibers in telecommunications are the refractive graded-index profile optical fibers. By minimizing the intermodal dispersion (i.e. the difference between the propagation delay times or group velocity of the optical modes along the optical fiber, also called DMGD for Differential Mode Group Delay), such a refractive index profile guaranties a high modal bandwidth for a given wavelength. However, such fiber designs unfortunately enhance modal coupling, which impedes the use of multimode fibers in the long distance communication scenario.
Since data traffic over fiber optic networks continues to grow exponentially, there is an increasing demand for increasing per-fiber traffic particularly across long distances. To this end, multiplexing techniques have been developed that allow a plurality of separate data streams to share the same optical fiber. Among these techniques, one promising approach is space division multiplexing (SDM), in which a plurality of data channels within a single optical fiber are provided by a respective plurality of optical signal modes guided by the fiber.
Such a technique has required the development of new types of optical fibers, called few-mode optical fibers, which support more than one spatial mode but fewer spatial modes than the multi-mode fibers. Such few-mode fibers, which are notably disclosed in the PCT patent document WO2011/094400, support approximately 2 to 50 modes. They can be configured so as to not have the modal dispersion problems that occur in multi-mode fibers.
Space-division-multiplexed transmissions using Few-Mode Fibers (FMFs) have hence recently received considerable attention because of their potential to multiply the capacity of single-mode transmissions by the number of modes that will be used.
One approach to the design of Few-Mode Fibers consists of minimizing the Differential Mode Group Delays (DMGDs, i.e. the difference in the respective arrival times of the guided modes used for spatial multiplexing), so that all modes can be simultaneously detected using complex 2N×2N (N being the total number of spatial modes, i.e. including LP (Linear Polarization) mode degeneracies) MIMO techniques, regardless mode-coupling phenomena that is one of the limiting factor to bridge long distances. This optimization, however, becomes more and more difficult when the number of LP modes increases.
It has to be noted, however, that less complex MIMO techniques may be used by grouping LP modes having close effective index differences, and detecting groups of LP modes, instead of individual LP modes.
A first known solution is disclosed in the US 2013/0071114 patent document, which describes a few mode optical fiber suitable for use in a mode division multiplexing optical transmission system. Such an optical fiber has a single alpha graded-index core with a radius R1 (with values up to 11.4 μm in the disclosed embodiments), an alpha value greater than or equal to about 2.3 and less than about 2.7 at a wavelength of 1550 nm, and a maximum relative refractive index Δ1MAX from about 0.3% to about 0.6% relative to the cladding. The optical fiber also has an effective area greater than about 90 μm2 and less than about 160 μm2. The cladding has a maximum relative refractive index Δ4MAX such that Δ1MAX>ΔΔMAX, and the differential group delay between the LP01 and LP11 modes is less than about 0.5 ns/km at a wavelength of 1550 nm.
However, according to this first known solution, the core and cladding support only the LP01 and LP11 modes at wavelengths greater than 1500 nm, which is a too small number of modes compared to the increasing demand on per-fiber transmission capacity.
A second known solution is disclosed in US 2013/007115, which disclose another specific design for Few-Mode Fibers. However, like the first known solution disclosed in US 2013/0071114, this second known-solution also consists in a FMF supporting only two guided modes.
Other known designs have led to FMFs supporting up to 4 or even 6 modes.
The PCT patent document WO 2012/161809 thus discloses a few-mode optical fiber comprising a core surrounded by a cladding, having a graded index profile that is structured to support propagation of a plurality of desired signal-carrying modes, while suppressing undesired modes. The core and cladding are configured such that the undesired modes have respective effective indices that are close to, or less than, the cladding index such that the undesired modes are leaky modes. The index spacing between the desired mode having the lowest effective index and the leaky mode with the highest effective index is sufficiently large so as to substantially prevent coupling therebetween. FMF supporting up to 4 modes are shown in examples.
The US 2012/0328255 patent document discloses few-mode optical fibers including a glass core and a glass cladding surrounding and in direct contact with the glass core. The glass core may include a radius R1i from about 8 μm to about 13 μm; a graded refractive index profile with an alpha value between about 1.9 and 2.1 at a wavelength of 1550 nm; and a maximum relative refractive index Δ1MAX from about 0.6% to about 0.95% relative to the glass cladding. The effective area of the LP01 mode at 1550 nm may be between 80 μm2 and 105 μm2 such that the core supports the propagation and transmission of an optical signal with X LP modes at a wavelength of 1550 nm, wherein X is an integer greater than 1 and less than 10. The glass cladding may include a maximum relative refractive index Δ4MAX such that Δ1MAX>ΔΔMAX. FMF supporting up to 6 modes are shown in examples.
Although such designs are promising, they do not allow reducing the Differential Mode Group Delays as much as desired, and therefore induce limits in the transmission system reach. In addition, the profiles disclosed in both documents are not optimized to ensure low bend losses and high leakage losses, which, however, are important issues for FMFs. Actually, none of the known documents relating to FMFs addresses the issue of designing a few-mode fiber showing low bend losses and high leakage losses.
Moreover, all these known techniques aim at providing new improved designs for few-mode fibers, and are based on theoretical criteria, which must be met by the fibers. However, these designs might be sensitive to small variations that can occur during the manufacturing process, such as profile variations, radius variations or refractive index difference variations for example. It is indeed very difficult to repeatedly manufacture optical fibers, which perfectly show the theoretical features, which were aimed at.
In the beginning of the 80s, a technique was proposed for multi-mode fibers, consisting in associating several optical fibers to form an optical link, where the optical link showed improved performance as compared to the individual optical fibers, thanks to a phenomenon of compensation of the fibers' imperfections. Such a technique has not yet been transposed to few-mode fiber links.
Accordingly, a need exists for design for few-mode optical fiber links guiding 4 LP modes or more, with small differential mode group delays, low bend losses and high leakage losses.